(a) Field of the Invention
This invention relates to methods for making polymeric articles and to the resulting articles. In particular it relates to a method of making a polymeric article from a blend comprising at least two polymers which are molecularly compatible and treating the article with a solvent.
(b) Information Disclosure
It is well known that most polymers are generally incompatible with each other. Most blends of two or more polymers contain the separate polymers as individual component domains or phases. Thus blends of what are termed compatible polymers generally are mechanically compatible only and exhibit properties which vary widely over the concentration range of the polymers. Such blends comprise a matrix polymer containing the other polymer as a dispersed or co-continuous phase. Such dispersed phases can be microscopic in size sometimes giving the resulting blend of multiple phases the appearance of being a single phase. There are, however, a few pairs of polymers which are molecularly compatible, that is they form a molecularly dispersed mixture comprising a single amorphous phase when they are blended together. Not only do such blends not separate into their individual amorphous components, but they are also characterized by having a single glass transition temperature (Tg). Mechanically compatible blends, on the other hand, exhibit two or more Tg's characteristic of the Tg's of the individual components. By the term glass transition temperature is meant the temperature at which an amorphous polymer or the amorphous regions of a partially crystalline polymer changes to or from a hard and relatively brittle state to a more flexible or rubbery condition. Measurement of glass transition temperatures of polymer systems is described, for example, in Thermal Characterization Techniques, Slade, et al., Marcel Dekker, Inc., New York (1970).
As mentioned above, very few polymeric blends are known wherein two or more polymers exhibit molecular compatibility. Reported in the literature are three basic classifications of compatible polymers represented by the following polymer pairs: poly(vinylidenefluoride) and poly(methyl methacrylate), poly(vinyl chloride) and poly(caprolactone), and poly(styrene) and poly-(phenyleneoxide). For examples of these type of compatible blends see Polymer Handbook, 2nd edition, Brandrup, et al. page III 211-213 which is incorporated herein by reference.
Blends of polyetherimides and poly(aryl ethers) are described in U.S. Pat. No. 4,293,670 to Robeson et al. incorporated herein by reference. The only specific blends reported in the patent are blends of a poly(aryl ether sulfone) and a poly(etherimide). Such blends exhibit mechanical compatibility with each blend having more than one glass transition temperature. Blends of a poly(etherimide) and a poly(aryl ether ketone) are discussed in general terms and no specific blend of a poly(aryl ether ketone) and a polyetherimide is reported. There is no mention that blends of these components would be molecularly compatible and therefore different from the poly(aryl ether sulfone)/polyetherimide blends specifically described.
Microporous articles prepared by blending one polymer with a pore forming additive which is then leached out, generally have pore sizes considerably larger than the pore sizes of the articles of this invention. When a polymer is utilized as the pore forming component to be leached from the polymer system, it had been thought that the polymers used should be at least partially incompatible. This is taught, for example, in U.S. Pat. No. 3,544,489 to Dowbenko, et al. In the Dowbenko, et al. patent a composition comprising a thermosetting resin and a solvent extractable thermoplastic resin are applied to a substrate, the thermosetting resin is cured resulting in the formation of minute discrete particles of the thermoplastic resin in the thermoset resin matrix. The thermoplastic is then extracted using a suitable solvent.
In U.S. Pat. No. 3,375,208, Duddy teaches the formation of microporous materials by treating a thermoplastic resin composition with a solvent which comprises intimately mixing under heat and pressure a particulate thermoplastic resin which is insoluble in said solvent with a substantially incompatible particulate thermoplastic resin which is soluble in said solvent to soften the resin into a plasticized mass, shaping the mass and thereafter contacting the shaped mass with said solvent to remove substantially all of the soluble thermoplastic resin and thereby render the insoluble thermoplastic resin microporous. It is specifically stated in U.S. Pat. No. 3,375,208 that it is the incompatibility of the resins and the intimate milling and blending thereof which results in a porous material having thread-like pores of capillary size.
In U.S. Pat. No. 4,096,099 to Koyama, et al., a porous film having fine cylindrical holes from 70 to 2000 A in diameter is disclosed. The resin film is prepared by treating a film of an AB or ABA type copolymer to decompose the copolymer to remove one of the components. It is clear that the polymer components used to make the block copolymers are incompatible with each other. Thus, the patentees resort to the use of block copolymers to obtain a microporous article in the desired film followed by decomposition of the block copolymer.